Location of Wideband OFDM Transmitters With Limited Receiver Bandwidth

ABSTRACT

One illustrative embodiment takes the form of a system for locating wireless transmitters employing an Orthogonal Frequency Division Multiplexing (OFDM) digital modulation scheme. The OFDM scheme comprises transmitting signal components over narrowband frequency channels spanning a wideband channel. The system includes a first receiving system configured to receive a fraction of the signal components transmitted by a first wireless transmitter to be located in a fraction of the narrowband frequency channels, and to process the fraction of the signal components to derive location related measurements. The system further includes at least a second receiving system configured to receive the fraction of the signal components transmitted by the first wireless transmitter, and to process this fraction of the signal components to derive location related measurements. The system also includes a processing system configured to use location related measurements from the first and second receiving systems to compute the location of the wireless transmitter.

TECHNICAL FIELD

The present invention relates generally to the field of wirelesscommunications, and more specifically to the location of wirelessdevices within the coverage area of a wireless communications network.Wireless devices, also called mobile stations (MS), include those suchas used in analog or digital cellular systems, personal communicationssystems (PCS), enhanced specialized mobile radios (ESMRs),wide-area-networks (WANs), and other types of wireless communicationssystems. This field is now generally known as wireless location, and hasapplication for Wireless E911, fleet management, RF optimization,security, and other valuable applications.

BACKGROUND

A. Wireless Location

Early work relating to Wireless Location Systems is described in U.S.Pat. No. 5,327,144, Jul. 5, 1994, “Cellular Telephone Location System,”which discloses a system for locating cellular telephones using timedifference of arrival (TDOA) techniques. This and other exemplarypatents (discussed below) are assigned to TruePosition, Inc., theassignee of the present invention.

The '144 patent describes what may be referred to as anuplink-time-difference-of-arrival (U-TDOA) cellular telephone locationsystem. The described system may be configured to monitor controlchannel transmissions from one or more cellular telephones and to usecentral or station-based processing to compute the geographiclocation(s) of the phone(s). For example, in station-based processing,which may be employed for reverse control channel signal detection,cross-correlations are performed at the cell sites (or signal collectionsystems) in the following manner: For each “strong” signal, which may beconsidered a reference signal, received on a particular control channelat a particular first cell site, that strong signal is first applied toa signal decoder, such as that used by the cellular system itself. Thisdecoder demodulates the cellular signal to produce the original digitalbit stream which had been modulated to produce the cellular signal. Thisdigital bit stream is then modulated by the cell site system toreconstruct the original signal waveform as it was first transmitted bythe cellular telephone. This reconstructed signal waveform iscross-correlated against the received signal at the first cell site. Thecross-correlation produces a peak from which an exact time of arrivalcan be calculated from a predetermined point on the peak. The first cellsite system then sends the demodulated digital bit stream and the timeof arrival to the central site over the communications line. The centralsite then distributes the demodulated digital bit stream and the exacttime of arrival to other cell sites likely to have also received thecellular transmission. At each of these other second, third, fourth,etc., cell sites, the digital bit stream is modulated by the cell sitesystem to reconstruct the original signal waveform as it was firsttransmitted by the cellular telephone. This reconstructed signalwaveform is cross-correlated against the signal received at each cellsite during the same time interval. The cross-correlation may or may notproduce a peak; if a peak is produced, an exact time of arrival (TOA)can be calculated from a predetermined point on the peak. This TOA isthen sent to the central site, and a delay difference, or TDOA, for aparticular pair of cell sites can be calculated. This method permits thecell site systems to extract TOA information from an extremely weaksignal reception, where the weak signal may be above or below the noiselevel. This method is applied iteratively to sufficient pairs of cellsites for each strong signal received at each cell site for each sampleperiod. The results of the delay pairs for each signal are then directedto the location calculation algorithm.

TruePosition and others (e.g., KSI, Inc.) have continued to developsignificant enhancements to the original inventive concepts. Someexamples are discussed below.

U.S. Pat. No. 6,047,192, Apr. 4, 2000, “Robust, Efficient, LocalizationSystem,” is another example of a prior art patent describing a similarprocess (referred to as “matched-replica processing”) for processingmobile transmitter signals to determine location related signalparameters, which may be employed to calculate the transmitter location.

Another exemplary prior art patent is U.S. Pat. No. 6,091,362, Jul. 18,2000, “Bandwidth Synthesis for Wireless Location System.” This patentdescribes a system and process offering improved accuracy of locationinformation and greater time resolution. In the described system,signals transmitted by wireless telephones are received at a pluralityof signal collection sites. To improve the accuracy of the locationinformation, the system synthesizes greater bandwidth, and thus greatertime resolution, than would otherwise be available. The location systemcan issue commands to cause a wireless transmitter to be located tochange frequency channels, and a doubly-differenced carrier phase of thetransmitted signal, or the TDOA, is observed at each of many frequenciesspanning a wide bandwidth. The phase-measurement data from these manyfrequencies are combined to resolve the inherent integer-wavelengthambiguity. The invention may be utilized to obtain a bandwidth greaterthan the typical bandwidth of the signals to be cross-correlated (ineither the time or frequency domains) in a cellular telephone locationapplication.

Another example is U.S. Pat. No. 6,646,604, Nov. 11, 2003, “AutomaticSynchronous Tuning of Narrowband Receivers of a Wireless Location Systemfor Voice/Traffic Channel Tracking.” This patent describes a transmitterlocating method that involves performing location processing on signalsreceived during an automatic sequential tuning mode of operation,wherein narrowband receivers are tuned sequentially and in unison to aplurality of predefined RF channels. Signal transmissions of interest inthese channels are digitally recorded and used in location processing.The identity of the located transmitter(s) is determined by matching alocation record to data indicating which wireless transmitters were inuse at a time corresponding to the location record, and which cell sitesand RF channels were used by each wireless transmitter.

An example of a wireless location system (WLS) of the kind describedabove is depicted in FIG. 1. As shown, the system includes four majorsubsystems: the Signal Collection Systems (SCS's) 10, the TDOA LocationProcessors (TLP's) 12, the Application Processors (AP's) 14, and theNetwork Operations Console (NOC) 16. Each SCS is responsible forreceiving the RF signals transmitted by the wireless transmitters onboth control channels and voice channels. In general, an SCS (nowsometimes called an LMU, or Location Measuring Unit) is preferablyinstalled at a wireless carrier's cell site, and therefore operates inparallel to a base station. Each TLP 12 is responsible for managing anetwork of SCS's 10 and for providing a centralized pool of digitalsignal processing (DSP) resources that can be used in the locationcalculations. The SCS's 10 and the TLP's 12 operate together todetermine the location of the wireless transmitters. Both the SCS's 10and TLP's 12 contain a significant amount of DSP resources, and thesoftware in these systems can operate dynamically to determine where toperform a particular processing function based upon tradeoffs inprocessing time, communications time, queuing time, and cost. Each TLP12 exists centrally primarily to reduce the overall cost of implementingthe WLS. In addition, the WLS may include a plurality of SCS regionseach of which comprises multiple SCS's 10. For example, “SCS Region 1”includes SCS's 10A and 10B that are located at respective cell sites andshare antennas with the base stations at those cell sites. Drop andinsert units 11A and 11B are used to interface fractional T1/E1 lines tofull T1/E1 lines, which in turn are coupled to a digital access andcontrol system (DACS) 13A. The DACS 13A and another DACS 13B are used inthe manner described more fully below for communications between theSCS's 10A, 10B, etc., and multiple TLP's 12A, 12B, etc. As shown, theTLP's are typically collocated and interconnected via an Ethernetnetwork (backbone) and a second, redundant Ethernet network. Alsocoupled to the Ethernet networks are multiple AP's 14A and 14B, multipleNOC's 16A and 16B, and a terminal server 15. Routers 19A and 19B areused to couple one WLS to one or more other Wireless Location System(s).

B. Evolving Wireless Standards and Air Interface Protocols

Over the past few years, the cellular industry has increased the numberof air interface protocols available for use by wireless telephones,increased the number of frequency bands in which wireless or mobiletelephones may operate, and expanded the number of terms that refer orrelate to mobile telephones to include “personal communicationsservices,” “wireless,” and others. The air interface protocols now usedin the wireless industry include AMPS, N-AMPS, TDMA, CDMA, GSM, TACS,ESMR, GPRS, EDGE, UMTS WCDMA, and others. UMTS is a wideband CDMA airinterface protocol defined by ETSI 3GPP. This protocol is similar to theCDMA protocols in EIA/TIA IS-95, or CDMA 2000, but does not requiresynchronization of the base stations, and also provides a high level ofinteroperability with GSM network infrastructure.

Orthogonal Frequency Division Multiplexing (OFDM) is a multiplexingtechnique in which a given subscriber may be assigned many frequencychannels over which it will simultaneously transmit. The multiplexingscheme provides high bandwidth efficiency and broadband wirelesscommunication in a high multi-path environment. WiFi as defined in IEEE802.11 and WiMax as defined in IEEE 802.16 utilize OFDM. It is expectedthat IEEE 802.20 (when re-ratified) will utilize OFDM.

Uplink TDOA location of fourth generation (4G) broadband OFDM signalswith bandwidths that can exceed 20 MHz requires expensive receiver andsignal processing hardware. The SCSs (or LMUs) may be required toreceive, sample, store and process these broadband signals, making thehardware significantly more expensive than what is required for thirdgeneration (3G) signals, such as UMTS or CDMA 2000 WCDMA signalsoccupying a bandwidth of 3-5 Mhz. As described in greater detail below,a goal of the present invention is to provide a way to accomplish U-TDOAlocation on the broadband 4G waveforms by collecting and processing onlya portion of the transmitted signal, reducing the required bandwidth,memory, and digital signal processing required in the SCS/LMU, whilestill achieving high accuracy.

SUMMARY

The following summary is intended to explain several aspects of theillustrative embodiments described in greater detail below. This summaryis not intended to cover all inventive aspects of the disclosed subjectmatter, nor is it intended to limit the scope of protection of theclaims set forth below.

One illustrative embodiment of the present invention takes the form of asystem for locating wireless transmitters employing an OrthogonalFrequency Division Multiplexing (OFDM) digital modulation scheme. TheOFDM scheme comprises transmitting signal components over a plurality ofpredefined narrowband frequency channels spanning a predefined widebandchannel. The system includes a first receiving system configured toreceive a fraction of the signal components transmitted by a firstwireless transmitter to be located in a fraction of the predefinednarrowband frequency channels, and to process the fraction of the signalcomponents to derive location related measurements. The system furtherincludes at least a second receiving system configured to receive thesaid fraction of the signal components transmitted by the first wirelesstransmitter, and to process this said fraction of the signal componentsto derive location related measurements. The system also includes aprocessing system configured to use location related measurements fromthe first and second receiving systems to compute the location of thewireless transmitter.

Other aspects of the embodiments disclosed herein are described below.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing summary as well as the following detailed description arebetter understood when read in conjunction with the appended drawings.For the purpose of illustrating the invention, there is shown in thedrawings exemplary constructions of the invention; however, theinvention is not limited to the specific methods and instrumentalitiesdisclosed. In the drawings:

FIG. 1 schematically depicts a Wireless Location System.

FIGS. 2A and 2B are block diagrams of the signal processing in OFDMtransmitters and receivers, respectively.

FIG. 3 illustrates a reduced spectrum processed by a SCS or LMU ascompared with the entire spectrum transmitted.

FIG. 4 schematically depicts an exemplary signal processing chainemployed by the SCSs of an illustrative embodiment.

FIG. 5 is a block diagram of a modified signal processing chain employedto support a reduced signal bandwidth.

FIG. 6 is a block diagram of a reconstruction process for the reducedsignal.

FIG. 7 is a flowchart of a station-based location process for thereduced signal.

FIG. 8A depicts an ideal cross-correlation function showing peaks due totwo signal components, a direct path component and a delayed componentdue to a multi-path reflection.

FIG. 8B depicts an ideal cross-correlation function (solid line) showingpeaks due to two signal components and a band-limited cross-correlationfunction showing the smearing of those peaks that make themindistinguishable.

FIG. 8C depicts an ideal cross-correlation function (solid line) showingpeaks due to two signal components and a band-limited cross-correlationfunction, with 4× the bandwidth of the function shown in FIG. 8B, stillshowing some smearing, but the increased bandwidth makes two individualpeaks distinguishable.

FIG. 9 illustrates the full bandwidth of an OFDM waveform with smallslices processed at any one time by an SCS/LMU, with multiple timeintervals used to cover most or all of the OFDM waveform bandwidth.

FIG. 10 schematically depicts a Wireless Location System for locatingOFDM transmitters in accordance with an illustrative embodiment.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

We will now describe illustrative or presently preferred embodiments ofthe present invention. First, we provide an overview and then a moredetailed description.

A. Overview

The present invention may be embodied in various forms, e.g., as asystem, method, or computer readable medium bearing executableinstructions for carrying out the inventive process. For example, asystem in accordance with the present invention may be implemented as asystem for locating wireless transmitters employing an OrthogonalFrequency Division Multiplexing (OFDM) digital modulation scheme. Theillustrative system is shown schematically in FIG. 10. The OFDM schemecomprises transmitting signal components over a plurality of narrowbandfrequency channels spanning a wideband channel. The system includesfirst and second receiving systems (elements 100 and 101 in FIG. 10),which may take the form of an SCS or LMU co-located at a basetransceiver station of a wireless communications system, although thisis by no means required. The receiving systems are each configured toreceive a fraction of the signal components transmitted by a wirelesstransmitter to be located (element 120 in FIG. 10) in a fraction of thenarrowband frequency channels, and to process the signal components toderive location related measurements. These measurements are thenprovided to a processing system (element 110) configured to use thelocation related measurements to compute the location of the wirelesstransmitter. The processing system may take the form of a TLP of thekind referred to above, although this is not required.

The location related measurements derived by the receiving systems maycomprise measurements of time difference of arrival (TDOA), time ofarrival (TOA), angle of arrival (AOA), round trip time, power, oranother form of measurement that may be used to calculate the locationof the wireless transmitter.

The fraction of the narrowband frequency channels received by thereceiving systems may include at least one pilot channel in which thewireless transmitter transmits energy, and the receiving systems may beconfigured to use signal components in the pilot channel to aid insignal acquisition and demodulation. Moreover, the fraction of thenarrowband frequency channels may exclude guard channels in which thewireless transmitter transmits minimal energy.

The receiving systems may each include a radio frequency (RF) filter,and they are preferably each configured to receive signal componentswithin a bandwidth compatible with the RF filter. The receiving systemsmay also include an intermediate frequency (IF) filter, and arepreferably configured to receive signal components within a bandwidthcompatible with the IF filter. In addition, the receiving systems mayeach include an analog to digital converter (ADC) characterized by asample rate, and are preferably configured to receive signal componentswithin a bandwidth compatible with the sample rate. The ADCs may becharacterized by a sample rate after decimation, and the receivingsystems may be configured to receive signal components within abandwidth compatible with the sample rate after decimation. Thereceiving systems may also include available memory for storing datarepresenting received signal components, and may be configured toreceive signal components within a bandwidth compatible with theavailable memory. The receiving systems may also include digital signalprocessors (DSPs) characterized by DSP processing throughput, and may beconfigured to receive signal components within a bandwidth compatiblewith the DSP processing throughput.

The receiving systems may be configured to receive signal componentswithin a bandwidth compatible with a current load on the receivingsystem. For example, the amount of DSP processing available within thereceiving system (e.g., SCS) at any point in time may be a function ofthe location processing load on the system. If the load happens to belower, and adequate DSP processing resources are available, then a widerportion of the transmitted bandwidth could be processed. However, if theload on the receiving system is high, a smaller portion of transmittedbandwidth would be processed to reduce the processing load on the DSPresources.

The receiving systems may also be configured to tune to a plurality ofchannels to receive signals from a plurality of wireless transmitters tobe located. In addition, the receiving systems may be configured to tuneto a plurality of selected channels, wherein the selected channels aredetermined based upon interference levels. For example, higherinterference could reduce the ability for receiving systems (e.g., LMUs)to detect signals, and could reduce the accuracy of computed locations.In general, it is better to select the portion of transmitted spectrumthat is least used by other transmitters. The level of interference overdifferent sections of the transmitted signal can be determined by makingpower measurements at the receiving system, and/or by using theknowledge of the channels used by other transmitters in the network. Thewireless network itself should have knowledge of the spectrumutilization.

The selected channels may be determined based upon various factors,including but not limited to measurements of received signals andspectrum usage.

A bandwidth synthesis process may also be advantageously employed inconnection with the present invention.

Moreover, use of the present invention may also involve use of asequential or random pattern of re-tuning a frequency agile receiver tocover most or all of the OFDM waveform spectrum.

In addition, a station-based or central processing method may beadvantageously used in practicing the invention.

B. Location of Broadband OFDM Transmitters with TDOA, Using Only aPortion of the Transmitted Spectrum

Broadband wireless communication infrastructure is being deployed andused on a large scale basis. WiFi capable devices, as defined in IEEE802.11 G, are capable communicating at rate of 54 mbps using a signalbandwidth on the order of 20 MHz. WiMAX capable devices as defined inIEEE 802.16 will be capable of communicating at rate of 75 mbps, withsignal bandwidth on the order of 20 MHz. This broadband capability willallow higher throughput applications to be used by wireless devices.Robust location techniques such as U-TDOA are needed for these mobiledevices for emergency and other location based services.

Orthogonal frequency-division multiplexing (OFDM), also sometimes calleddiscrete multitone modulation (DMT), is based upon the principle offrequency-division multiplexing (FDM), but is often used as a digitalmodulation scheme. The bit stream that is to be transmitted is splitinto several parallel bit streams, typically dozens to thousands, andthe available frequency spectrum is divided into several sub-channels,and each low-rate bit stream is transmitted over one sub-channel bymodulating a sub-carrier using a standard modulation scheme, for examplePSK, QAM, etc. The sub-carrier frequencies are chosen so that themodulated data streams are orthogonal to each other, meaning thatcross-talk between the sub-channels is eliminated. Channel equalizationis simplified by using many slowly modulated narrowband signals insteadof one rapidly modulated wideband signal. An advantage of OFDM is itsability to cope with severe channel conditions, such as multipath andnarrowband interference, without complex equalization filters. Asmentioned, OFDM has developed into a popular scheme for wideband digitalcommunication systems.

In OFDM, the sub-carrier frequencies are chosen so that the modulateddata streams are orthogonal to each other, meaning that cross-talkbetween the sub-channels is eliminated and inter-carrier guard bands arenot required. This greatly simplifies the design of both the transmitterand the receiver without a separate filter for each sub-channel, whichis required in conventional FDM. The orthogonality also allows highspectral efficiency, near the Nyquist rate. The orthogonality alsoallows for efficient modulator and demodulator implementation using theFFT algorithm. Although the principles and some of the benefits havebeen known since the 1960s, OFDM is made popular today for widebandcommunication by availability of low-cost digital signal processingcomponents that can efficiently calculate the FFT. OFDM requiresaccurate frequency synchronization in the receiver; any inaccuracy meansthat the sub-carriers no longer appear orthogonal, resulting in degradedperformance.

U-TDOA location of devices transmitting these signals becomes achallenge, as receivers are needed to capture very high bandwidthsignals, store and process them. The requirements for RF signalbandwidth, digital signal processing power, and memory required toperform U-TDOA location on a signal with a 20 MHz bandwidth signal canbe six times that required to locate third generation (3G) wirelessdevices utilizing signals with a bandwidth of 3 to 5 MHz. Theseincreased requirements can dramatically increase the cost and complexityof the Signal Collection System or LMU (the terms SCS and LMU will beused interchangeably herein).

With an embodiment of the present invention, TDOA location of abroadband wireless transmitter is accomplished by selecting only aportion of the spectrum of the transmitted signal, which can besupported by the available capability of the SCSs measuring the signal.The capability includes the level of receiver bandwidth, signal samplingrate, DSP processing throughput, and memory. As an example, a SCS may beequipped with an RF receiver containing filters with sufficientbandwidth to support a 3GPP UMTS waveform (3-5 MHz bandwidth), analog todigital converters capable of sampling a 3-5 MHz wide signal, anddigital signal processing resources and memory capable of performingTDOA location processing of a signal with 3-5 MHz of bandwidth, but withthe SCS incapable of collecting and processing a full 20 MHz bandwidthsignal. In this case, a contiguous portion of the transmitted signal maybe selected, with this portion having a signal bandwidth that is withinthe capabilities of the SCS. This signal reduction is possible becausethe OFDM waveform transmitted by a broadband device actually consists ofmany (256 for example) contiguous channels, which can be individuallydemodulated and separated from the rest of the signal. A block of 64channels, which might be selected to be a power of 2 for FFT efficiency,may be processed in the TDOA location computation. In a direct sequencespread spectrum system such as IS-95, or UMTS, this would not bepossible, as there would be no way to extract any meaningful data from asmall portion of the transmitted signal. A small portion of the spectrumcould not be demodulated without the rest of the signal as in an OFDMwaveform. Because these are high bandwidth signals, a station-basedprocess as defined in the '144 patent could be used as this minimizesthe amount of data transferred, although signal data could betransferred to a central node for central correlation processing, asalso described in the '144 patent. This technique applies to bothwideband and narrowband embodiments of the SCS.

The transmitted waveform used in the IEEE 802.16 WiMAX system consistsof 256 channels. The outer 55 channels are guard channels in whichminimal energy is transmitted. In addition, there are 8 pilot channelsto aid in signal acquisition and demodulation. Selection of thebandwidth to process should include a number of pilot channels which areplaced through the full channel set to help the receiver properly detectand demodulate the signal. In addition, the guard signals are goodcandidates to exclude as they contain little useful signal energy. Thechannel set selection could also be based upon knowledge of the currentutilization of the spectrum, where less utilized spectrum is chosen forprocessing to minimize the likelihood of interference. The selectedchannel set may also be chosen to be a power of 2 or 4 to allow forefficient multiplexing with an FFT.

A transmitted OFDM waveform is typically constructed as shown in FIG.2A. The process may be summarized as follows:

-   -   1. Information bits are encoded with additional redundant and        parity bits to allow the receiver to detect and correct errors.        (Reference numeral 20.)    -   2. Data are interleaved to distribute the redundant bits over a        larger time to allow the redundancy in the error correction        codes to correct short degradations in received signal quality.        (Reference numeral 21.)    -   3. The encoded bits are modulated into PSK or QAM symbols, in        the form of base-band sample data. (Reference numeral 22.)    -   4. A block of PSK or QAM symbols (256) are passed through an        inverse Fast Fourier Transform (IFFT) creating the OFDM signal.        (Reference numeral 23.)    -   5. The digital signal is then converted to analog with a digital        to analog converter. (Reference numeral 24.)    -   6. The signal is frequency converted to Radio Frequency (RF) and        then it is transmitted. (Reference numeral 25.)

A typical OFDM receiver performs the following steps shown in FIG. 2B.This process is essentially the reverse of the transmitter process:

-   -   1. RF signal is frequency converted to base-band, filtered, and        digitized. (Reference numerals 26 and 27.) This may include:        -   a. One more stages of frequency conversion of the analog            signal to and intermediate frequency (IF), or base-band;        -   b. Filtering of the analog signal to a bandwidth which            satisfies the Nyquist criteria for the signal bandwidth, and            sample rate;        -   c. Digitizing base-band or IF signal with an analog to            digital converter;        -   d. Digital down-conversion of IF to base-band if necessary;            and        -   e. Possible additional digital filtering, and decimation to            a lower sample rate.    -   2. Receiver performs an FFT of a block (256) of samples, which        converts the OFDM signal into a single channel high data rate        signal. (Reference numeral 28.)    -   3. Receiver demodulates PSK or QAM signals and outputs coded        bits. (Reference numeral 29.)    -   4. Signal is de-interleaved. (Reference numeral 30.)    -   5. Encoded bits are decoded, providing original information        bits. (Reference numeral 31.)

FIG. 3 shows how only a portion of the transmitted channels of an OFDMsignal is selected for location processing.

FIG. 4 shows the signal processing chain of the SCS. In an illustrativeexample of the present invention, the SCS has RF signals from antennasconnected to the input. These RF signals may contain some undesired outof band signals from the base station transmitter, or other interferers.The RF filter 40 reduces the levels of the undesired signals outside ofthe pass-band of the desired signals, while allowing the pass-bandsignals to pass to the next stage with minimal loss. The filtered RFsignal is then frequency converted 41 to an IF frequency of around 70MHz. The frequency conversion process is accomplished by modulating theRF signal with a sinusoidal Local Oscillator (LO) signal with afrequency about 70 MHz lower than the desired RF frequency. This willcause the RF signal to be translated to frequency around 70 Hz.Adjusting the LO frequency will allow different portions of the LOfrequency to be tuned around 70 MHz. In this case the desired portion ofthe receiver RF signal will be tuned to a center frequency of 70 MHz.

The IF signal is then passed through an IF filter 42 to reduce thebandwidth of the signal such that it can easily be sampled at a ratemeeting the Nyquist criteria to avoid aliasing. The IF filter 42 has apass-band of 5 MHz and a center frequency of 70 MHz. The filter, whichcould be made up of one or more cascaded surface acoustic wave (SAW)filters, reduces the power level of all signals outside of a 10 MHzbandwidth by 75 dB, relative to the pass-band level. A filter of thistype is selected because many transceivers are designed with a 70 MHz IFfrequency, and filters with a 5 MHz pass-band are commonly used in WCDMAand cable TV equipment. These filters are inexpensive and readilyavailable. Passing a wider bandwidth of 20 MHz would likely require acustom filter design, and increase the SCS cost. The filtered IF signalis then sampled by the analog to digital converter 43, with a samplerate of 60 MSPS. A high sampling rate permits the use of digital downconverters with output signal sample rates of ˜12 MSPS. The digitizedsignal is then passed through a digital down converter 44, where thedigital signal is filtered to a bandwidth of <5 MHz, and converted fromIF to base-band. In this process the sample rate is also decimated to 12MSPS. The decimation eliminates the redundant samples, reducing theprocessing load on the DSP 45.

The largest savings from reducing the processes spectrum is in memoryand DSP processing throughput. The required memory and DSP throughputcan be compared when performing a TDOA measurement on the full 20 MHzsignal, which would have a sample rate of 48 MHz vs. a 5 MHz portion ofthe signal, with a sample rate of 12 MHz. TDOA measurements are made byperforming a cross correlation of the signal received by one SCS with areference signal received at another SCS, as a function of timedifference, as shown below.

${y(\tau)} = {\sum\limits_{N}{{x(\tau)}{r\left( {n + \tau} \right)}}}$

where x(n) is the received signal, r(n) is the reference signal, and Nis the number of samples in both the received and reference signals.

Both the size of memory required to store the signal, as well as thenumber of multiplications to perform the correlation are a linearfunction of the number of samples. If one second of received andreference data are to be used for correlation, the full signal wouldrequire the storage of 48 million samples, and 48 millionmultiplications to compute a single cross-correlation value. The reducedsignal would require storage of 12 million samples, and 12 millionmultiplications to compute a single correlation value. The reducedsignal requires only ¼ the memory and DSP 45 power as the full signal.

FIG. 5 shows the demodulation and decoding by the primary SCS in astation-based processing implementation. Because much of the underlyingdata is missing due to the reduction in processed spectrum, the steps ofinterleaving and decoding are not feasible, and are eliminated, furtherreducing required processing.

-   -   1. RF signal is frequency converted to base-band, filtered, and        digitized. (Reference numerals 50 and 51.) This may include:        -   a. One more stages of frequency conversion of the analog            signal to and intermediate frequency (IF), or baseband;        -   b. Filtering of the analog signal to a bandwidth which            satisfies the Nyquist criteria for the signal bandwidth, and            sample rate,            -   i. The sample rate is be much lower than the 48 MHz                required to properly sample a 20 MHz signal;            -   ii. The filter bandwidth could be much less than the 20                MHz required to pass an entire signal;        -   c. Digitizing base-band or IF signal with an analog to            digital converter;        -   d. Digital down-conversion of IF to base-band if necessary;        -   e. Possible additional digital filtering, and decimation to            a lower sample rate;    -   2. FFT performed on of a block (64) of samples. (Reference        numeral 52.)    -   3. PSK or QAM signals demodulated to coded bits. (Reference        numeral 53.)

The reconstruction process used for the reduced signal is shown in FIG.6.

-   -   1. The encoded bits are modulated into PSK or QAM symbols, in        the form of base-band sample data. (Reference numeral 60.)    -   2. A block of PSK or QAM symbols (256) are passed through an        inverse Fast Fourier Transform (IFFT) creating the OFDM signal.        (Reference numeral 61.)    -   3. Additional characteristics are applied to the signal, such as        phase corrections. (Not shown.)

Therefore, the station-based TDOA location process for the reducedwaveform would be as shown in FIG. 7:

-   -   1. The primary SCS, as well as cooperating SCSs receive and        digitize the transmitted signal (reference numeral 70):        -   a. Sampling of the receive signals is synchronized to            facilitate TDOA processing.        -   b. Sampled signal bandwidth and sample rate may be reduced,            as only a fraction of the signal bandwidth will be            processed.    -   2. The primary SCS implements the demodulation steps above,        which excludes de-interleaving and error correction decoding,        and also measures other signal characteristics, such as phase        corrections. (Reference numeral 71.)    -   3. Encoded bits and characteristic data is transferred to        cooperating SCSs. (Reference numeral 72.)    -   4. Primary and cooperating SCSs reconstruct the reference        base-band signal, by implementing the steps shown in FIG. 6.        (Reference numeral 73.)    -   5. Primary and cooperating SCSs perform correlation processing        to measure the Time Difference of Arrival of the signal, and        send the TDOA measurement to the TLP. (Reference numeral 74.)    -   6. TLP computes the location. (Reference numeral 75.)

The concepts described herein are not limited to WiFi or WiMAX systems,but apply to any system which uses OFDM for communication. The inventionis not limited to the specific architecture and/or implementationdefined for the SCS.

Alternate Embodiments

An extension to the above approach allows the use of a narrower-bandfront end to capture just a portion of the OFDM waveform spectrum asdescribed above, while maintaining the improved multi-path resolutionthat can be achieved using the wider-band waveform that is transmittedby the mobile device. This extension involves sampling a portion of theOFDM waveform spectrum as described above for a interval of time, thenre-tuning the frequency agile receiver to sample a different portion ofthe OFDM waveform spectrum in the next interval of time, then continuingthis process to get multiple slices of the OFDM waveform spectrum (up tocovering the entire OFDM waveform spectrum with a series of narrow-bandsamples). This re-tuning can be performed in sequential or randompatterns to cover most or all of the OFDM waveform bandwidth. This isillustrated in FIG. 9. (See also, U.S. Pat. No. 6,646,604, Nov. 11,2003, “Automatic Synchronous Tuning of Narrowband Receivers of aWireless Location System for Voice/Traffic Channel Tracking,” which ishereby incorporated by reference in its entirety.)

The ability to resolve multi-path components in the cross-correlationfunction used to measure TDOA values is limited by the bandwidth of thesignal that is used. When there is a direct path signal and a delayedsignal that arrives in close proximity in time, an ideal correlationfunction using infinite bandwidth signals would result in two peaks thatare easily resolvable as shown in FIG. 8A. When band limited signals areused to generate the cross-correlation function, these peaks are“smeared” by a smoothing function whose width is proportional to theinverse of the bandwidth of the signal. When this inverse bandwidth iswider then the separation between the arriving signals, they becomeindistinguishable as shown in FIG. 8B. If, however, this inversebandwidth is narrower then the separation between the arriving signals,then the peaks in the correlation function, while still smeared, can beeasily distinguished, as shown in FIG. 8C, where the bandwidth is 4times that of the signal in FIG. 8B. The ability to distinguish thedifferent signal arrivals allows the selection of the direct pathsignal. This provides a more accurate TDOA measurement, directlyreducing error of the location estimate.

This advantage of a wider bandwidth cross-correlation function can beachieved without the added cost of sampling the full bandwidthsimultaneously, which would require a wider-band front receiver, highersample rate A/D converter, more storage, and processing power. Instead,a series of narrow-band samples can be stored, and the advantage of thewider bandwidth cross-correlation function can be achieved using thebandwidth synthesis process described in U.S. Pat. No. 6,091,362, Jul.18, 2000, “Bandwidth Synthesis for Wireless Location System,” which ishereby incorporated by reference in its entirety.

In frequency hopped waveforms such as GSM, the advantage gained byperforming bandwidth synthesis can be somewhat limited by the fact thatthe spacing of the sampled frequency is not contiguous in general, andcan be quite sparse in practice. This sparse spacing results inambiguities in the synthesized cross-correlation function that may notbe successfully resolved. In this embodiment, the OFDM waveform occupiesa large contiguous block of spectrum which is sampled using a series ofnarrower slices of the spectrum. This insures that the slices will beadjacent to each other in frequency (see FIG. 9), allowing the bandwidthsynthesis process to produce a synthesized cross-correlation functionthat does not contain ambiguities.

C. Conclusion

The true scope the present invention is not limited to the presentlypreferred embodiments disclosed herein. For example, the foregoingdisclosure of a presently preferred embodiment of a Wireless LocationSystem uses explanatory terms, such as Signal Collection System (SCS),TDOA Location Processor (TLP), Applications Processor (AP), LocationMeasuring Unit (LMU), and the like, which should not be construed so asto limit the scope of protection of the following claims, or tootherwise imply that the inventive aspects of the Wireless LocationSystem are limited to the particular methods and apparatus disclosed.Moreover, as will be understood by those skilled in the art, many of theinventive aspects disclosed herein may be applied in location systemsthat are not based on TDOA techniques. For example, the invention is notlimited to systems employing SCS's constructed as described above. TheSCS's, TLP's, etc. are, in essence, programmable data collection andprocessing devices that could take a variety of forms without departingfrom the inventive concepts disclosed herein. Given the rapidlydeclining cost of digital signal processing and other processingfunctions, it is easily possible, for example, to transfer theprocessing for a particular function from one of the functional elements(such as the TLP) described herein to another functional element (suchas the SCS) without changing the inventive operation of the system. Inmany cases, the place of implementation (i.e., the functional element)described herein is merely a designer's preference and not a hardrequirement. Accordingly, except as they may be expressly so limited,the scope of protection of the following claims is not intended to belimited to the specific embodiments described above.

1. A system for locating wireless transmitters employing an OrthogonalFrequency Division Multiplexing (OFDM) digital modulation scheme,wherein said OFDM scheme comprises transmitting signal components over aplurality of predefined narrowband frequency channels spanning apredefined wideband channel, comprising: a first receiving systemconfigured to receive a fraction of the signal components transmitted bya first wireless transmitter to be located in a fraction of thepredefined narrowband frequency channels, and to process said fractionof the signal components to derive location related measurements; asecond receiving system configured to receive said fraction of thesignal components transmitted by said first wireless transmitter, and toprocess said fraction of the signal components to derive locationrelated measurements; and a processing system, operatively coupled tosaid first and second receiving systems, wherein said processing systemis configured to use location related measurements from said first andsecond receiving systems to compute the location of said wirelesstransmitter.
 2. A system as recited in claim 1, wherein said locationrelated measurements derived by said first and second receiving systemscomprise time difference of arrival (TDOA) measurements.
 3. A system asrecited in claim 1, wherein said location related measurements derivedby said first and second receiving systems comprise time of arrival(TOA) measurements.
 4. A system as recited in claim 1, wherein saidlocation related measurements derived by said first and second receivingsystems comprise angle of arrival (AOA) measurements.
 5. A system asrecited in claim 1, wherein said location related measurements derivedby said first and second receiving systems comprise round trip timemeasurements.
 6. A system as recited in claim 1, wherein said locationrelated measurements derived by said first and second receiving systemscomprise received power measurements.
 7. A system as recited in claim 1,wherein said fraction of the predefined narrowband frequency channelsincludes at least one pilot channel in which said first wirelesstransmitter transmits energy, and wherein said first or second receivingsystems are configured to use signal components in said pilot channel toaid in signal acquisition and demodulation.
 8. A system as recited inclaim 1, wherein said fraction of the predefined narrowband frequencychannels excludes guard channels in which said first wirelesstransmitter transmits minimal energy.
 9. A system as recited in claim 1,wherein said first and second receiving systems each include a radiofrequency (RF) filter, and wherein the first and second receivingsystems are each configured to receive signal components within abandwidth compatible with said RF filter.
 10. A system as recited inclaim 1, wherein said first and second receiving systems each include anintermediate frequency (IF) filter, and wherein the first and secondreceiving systems are each configured to receive signal componentswithin a bandwidth compatible with said IF filter.
 11. A system asrecited in claim 1, wherein said first and second receiving systems eachinclude an analog to digital converter (ADC) characterized by a samplerate, and wherein the first and second receiving systems are eachconfigured to receive signal components within a bandwidth compatiblewith said sample rate.
 12. A system as recited in claim 11, wherein saidADCs are further characterized by a sample rate after decimation, andwherein the first and second receiving systems are configured to receivesignal components within a bandwidth compatible with said sample rateafter decimation.
 13. A system as recited in claim 1, wherein said firstand second receiving systems each include available memory for storingdata representing received signal components, and wherein said first andsecond receiving systems are configured to receive signal componentswithin a bandwidth compatible with said available memory.
 14. A systemas recited in claim 1, wherein said first and second receiving systemseach include digital signal processors (DSPs) characterized by DSPprocessing throughput, and wherein said first and second receivingsystems are configured to receive signal components within a bandwidthcompatible with said DSP processing throughput.
 15. A system as recitedin claim 1, wherein said first and second receiving systems areconfigured to receive signal components within a bandwidth compatiblewith a current load on the receiving system.
 16. A system as recited inclaim 1, wherein said first and second receiving systems are configuredto tune to a plurality of channels to receive signals from a pluralityof wireless transmitters to be located.
 17. A system as recited in claim1, wherein said first and second receiving systems are configured totune to a plurality of selected channels, wherein said selected channelsare determined based upon interference levels.
 18. A system as recitedin claim 17, wherein said selected channels are determined based uponmeasurements of received signals.
 19. A system as recited in claim 17,wherein said selected channels are determined based upon spectrum usage.20. A system as recited in claim 1, wherein said first and secondreceiving systems are configured for collection of multiple slices ofspectrum for bandwidth synthesis.
 21. A system as recited in claim 1,wherein said first and second receiving systems are configured for useof a sequential pattern of re-tuning a frequency agile receiver to covera substantial part of an OFDM waveform spectrum.
 22. A system as recitedin claim 1, wherein said system is configured for use of a bandwidthsynthesis process to provide increased resolution in a cross-correlationfunction to reduce multipath delay spread.
 23. A system as recited inclaim 1, wherein said system is configured to use a station-basedprocessing method.
 24. A system as recited in claim 1, wherein saidfirst or second receiving systems include means for performing OFDMreceiving and demodulation processing.
 25. A system as recited in claim1, wherein said first and second receiving systems include narrowbandreceivers.
 26. A system as recited in claim 1, wherein said first andsecond receiving systems include wideband receivers.
 27. A method forlocating wireless transmitters employing an Orthogonal FrequencyDivision Multiplexing (OFDM) digital modulation scheme, wherein saidOFDM scheme comprises transmitting signal components over a plurality ofnarrowband frequency channels spanning a wideband channel, comprising:at a first receiving system, receiving a fraction of the signalcomponents transmitted by a first wireless transmitter to be located ina fraction of the narrowband frequency channels, and processing saidfraction of the signal components to derive location relatedmeasurements; at a second receiving system, receiving said fraction ofthe signal components transmitted by said first wireless transmitter,and processing said fraction of the signal components to derive locationrelated measurements; and processing said location related measurementsfrom said first and second receiving systems to compute the location ofsaid wireless transmitter.
 28. A method as recited in claim 27, whereinsaid location related measurements comprise time difference of arrival(TDOA) measurements.
 29. A method as recited in claim 27, wherein saidlocation related measurements comprise time of arrival (TOA)measurements.
 30. A method as recited in claim 27, wherein said locationrelated measurements comprise angle of arrival (AOA) measurements.
 31. Amethod as recited in claim 27, wherein said location relatedmeasurements comprise round trip time measurements.
 32. A method asrecited in claim 27, wherein said location related measurements comprisereceived power measurements.
 33. A method as recited in claim 27,wherein said fraction of the narrowband frequency channels includes atleast one pilot channel in which said first wireless transmittertransmits energy, and wherein signal components in said pilot channelare used to aid in signal acquisition and demodulation.
 34. A method asrecited in claim 27, wherein said fraction of the narrowband frequencychannels excludes guard channels in which said first wirelesstransmitter transmits minimal energy.
 35. A method as recited in claim27, wherein said first and second receiving systems each include a radiofrequency (RF) filter, and wherein the method includes receiving signalcomponents within a bandwidth compatible with said RF filter.
 36. Amethod as recited in claim 27, wherein said first and second receivingsystems each include an intermediate frequency (IF) filter, and whereinthe method includes receiving signal components within a bandwidthcompatible with said IF filter.
 37. A method as recited in claim 27,wherein said first and second receiving systems each include an analogto digital converter (ADC) characterized by a sample rate, and whereinthe method includes receiving signal components within a bandwidthcompatible with said sample rate.
 38. A method as recited in claim 37,wherein said ADCs are further characterized by a sample rate afterdecimation, and wherein the method includes receiving signal componentswithin a bandwidth compatible with said sample rate after decimation.39. A method as recited in claim 27, wherein said first and secondreceiving systems each include available memory for storing datarepresenting received signal components, and wherein the method includesreceiving signal components within a bandwidth compatible with saidavailable memory.
 40. A method as recited in claim 27, wherein saidfirst and second receiving systems each include digital signalprocessors (DSPs) characterized by DSP processing throughput, andwherein the method includes receiving signal components within abandwidth compatible with said DSP processing throughput.
 41. A methodas recited in claim 27, wherein the method includes, at the firstreceiving system, receiving signal components within a bandwidthcompatible with a current load on the first receiving system.
 42. Amethod as recited in claim 27, wherein the method includes, at the firstand second receiving systems, tuning to a plurality of channels toreceive signals from a plurality of wireless transmitters to be located.43. A method as recited in claim 27, wherein the method includes, at thefirst and second receiving systems, tuning to a plurality of selectedchannels, wherein said selected channels are determined based uponinterference levels.
 44. A method as recited in claim 43, wherein saidselected channels are determined based upon measurements of receivedsignals.
 45. A method as recited in claim 43, wherein said selectedchannels are determined based upon spectrum usage.
 46. A method asrecited in claim 27, wherein the method further comprises, at the firstand second receiving systems, collection of multiple slices of spectrumfor bandwidth synthesis.
 47. A method as recited in claim 27, whereinthe method further comprises, at the first and second receiving systems,use of a sequential pattern of re-tuning a frequency agile receiver tocover a substantial part of an OFDM waveform spectrum.
 48. A method asrecited in claim 27, wherein the method further comprises, at the firstand second receiving systems, use of a bandwidth synthesis process toprovide increased resolution in a cross-correlation function to reducemultipath delay spread.
 49. A method as recited in claim 27, furthercomprising the use a station-based processing method.
 50. A method asrecited in claim 27, wherein the method further comprises, at the firstor second receiving systems, performing OFDM receiving and demodulationprocessing.
 51. A method as recited in claim 27, wherein the methodfurther comprises, at the first and second receiving systems, the use ofnarrowband receivers.
 52. A method as recited in claim 27, wherein themethod further comprises, at the first and second receiving systems, theuse of wideband receivers.
 53. A computer readable medium comprisingexecutable instructions for use by a receiving system deployed in asystem for locating wireless transmitters employing an OrthogonalFrequency Division Multiplexing (OFDM) digital modulation scheme,wherein said OFDM scheme comprises transmitting signal components over aplurality of predefined narrowband frequency channels spanning apredefined wideband channel, the computer readable medium comprisingexecutable instructions for causing a receiving system to carry out amethod comprising the steps of receiving a fraction of the signalcomponents transmitted by a wireless transmitter to be located in afraction of the narrowband frequency channels, and processing saidfraction of the signal components to derive location relatedmeasurements.
 54. A computer readable medium as recited in claim 53,wherein said location related measurements comprise time difference ofarrival (TDOA) measurements.
 55. A computer readable medium as recitedin claim 53, wherein said location related measurements comprise time ofarrival (TOA) measurements.
 56. A computer readable medium as recited inclaim 53, wherein said location related measurements comprise angle ofarrival (AOA) measurements.
 57. A computer readable medium as recited inclaim 53, wherein said location related measurements comprise round triptime measurements.
 58. A computer readable medium as recited in claim53, wherein said location related measurements comprise received powermeasurements.
 59. A computer readable medium as recited in claim 53,wherein said fraction of the narrowband frequency channels includes atleast one pilot channel in which said first wireless transmittertransmits energy, and wherein signal components in said pilot channelare used to aid in signal acquisition and demodulation.
 60. A computerreadable medium as recited in claim 53, wherein said fraction of thenarrowband frequency channels excludes guard channels in which saidfirst wireless transmitter transmits minimal energy.
 61. A computerreadable medium as recited in claim 53, wherein said receiving systemincludes a radio frequency (RF) filter, and wherein the method includesreceiving signal components within a bandwidth compatible with said RFfilter.
 62. A computer readable medium as recited in claim 53, whereinsaid receiving system includes an intermediate frequency (IF) filter,and wherein the method includes receiving signal components within abandwidth compatible with said IF filter.
 63. A computer readable mediumas recited in claim 53, wherein said receiving system includes an analogto digital converter (ADC) characterized by a sample rate, and whereinthe method includes receiving signal components within a bandwidthcompatible with said sample rate.
 64. A computer readable medium asrecited in claim 63, wherein said ADCs are further characterized by asample rate after decimation, and wherein the method includes receivingsignal components within a bandwidth compatible with said sample rateafter decimation.
 65. A computer readable medium as recited in claim 53,wherein said receiving system includes available memory for storing datarepresenting received signal components, and wherein the method includesreceiving signal components within a bandwidth compatible with saidavailable memory.
 66. A computer readable medium as recited in claim 53,wherein said receiving system includes digital signal processors (DSPs)characterized by DSP processing throughput, and wherein the methodincludes receiving signal components within a bandwidth compatible withsaid DSP processing throughput.
 67. A computer readable medium asrecited in claim 53, wherein the method includes, at the receivingsystem, receiving signal components within a bandwidth compatible with acurrent load on the receiving system.
 68. A computer readable medium asrecited in claim 53, wherein the method includes, at the receivingsystem, tuning to a plurality of channels to receive signals from aplurality of wireless transmitters to be located.
 69. A computerreadable medium as recited in claim 53, wherein the method includes, atthe receiving system, tuning to a plurality of selected channels,wherein said selected channels are determined based upon interferencelevels.
 70. A computer readable medium as recited in claim 69, whereinsaid selected channels are determined based upon measurements ofreceived signals.
 71. A computer readable medium as recited in claim 69,wherein said selected channels are determined based upon spectrum usage.72. A computer readable medium as recited in claim 53, wherein themethod further comprises, at the receiving system, collection ofmultiple slices of spectrum for bandwidth synthesis.
 73. A computerreadable medium as recited in claim 53, wherein the method furthercomprises, at the receiving system, use of a sequential pattern ofre-tuning a frequency agile receiver to cover a substantial part of anOFDM waveform spectrum.
 74. A computer readable medium as recited inclaim 53, wherein the method further comprises, at the receiving system,use of a bandwidth synthesis process to provide increased resolution ina cross-correlation function to reduce multipath delay spread.
 75. Acomputer readable medium as recited in claim 53, wherein the methodfurther comprises the use a station-based processing method.
 76. Acomputer readable medium as recited in claim 53, wherein the methodfurther comprises, at the receiving system, performing OFDM receivingand demodulation processing.
 77. A computer readable medium as recitedin claim 53, wherein the method further comprises, at the receivingsystem, the use of narrowband receivers.
 78. A computer readable mediumas recited in claim 53, wherein the method further comprises, at thereceiving system, the use of wideband receivers.